There have been a number of signs of spring wedged in between the cold and the snows. For the past two weeks the mornings have been greeted by a chorus of bird songs no matter what the temperature or type of on-going precipitation. The male cardinals, titmice, Carolina wrens, mourning doves, and robins are all showing off their individual energies and potential reproductive fitnesses by shaking off their cold night’s sleep with loud, echoing songs. Also, the common grackles have returned to my front yard. The male grackles in between beaks-full of shelled corn and long drinks from my bird bath, dance slowly around each other (and the smaller, more drab colored females) with ruffed out neck feathers and sky-pointed beaks. I have been making sure that my bird feeder area is well stocked early in the morning for all of the day’s action.
The branches of my spruce trees have been lively, too, with pairs (and even trios!) of titmice and cardinals chasing each other about trying to be noticed. The gray and red squirrels have been running up and down the tree trunks, too, much to the joy of my still mostly house-bound cats.
But the biggest sign of spring to date was something that Deborah, Rob, Michele and I saw Saturday on our hike around Ferncliff Peninsula down at Ohiopyle. Not only did we see colt’s foot flowers (at last!) scattered about in the warmer areas of the trail and a comma butterfly fluttering around under the rhododendrons, but we also saw eastern wood frogs gathering in loud, chirping numbers in most of the vernal pools along the trail.
Video: D. Sillman
The eastern wood frog (Rana sylvatica) is found from northern Georgia all the way up to the Arctic Circle. In fact, it is the only “cold blooded” vertebrate known to live north of the Arctic Circle! They utilize temporary pools formed by spring rains and snow melt as breeding pools and then spend most of the rest of their active season away from standing water. Adult wood frogs feed opportunistically and extensively on small insects and other invertebrates. They use their long, sticky tongues to capture prey and are said to eat “anything that they can fit into their mouths.”
The ability of the wood frog to survive in high latitude ecosystems depends upon a number of specialized physiological adaptations that include the presence of specific proteins in the blood that regulate ice crystal formation, circulatory controls that shunt blood preferentially into critical organs (heart, liver, brain), and a liver response that releases huge amounts of glucose into the blood stream and organs. These high levels of glucose act as a cryoprotectant which reduces the amount of ice formed in the protected tissues and cells.
During hibernation, 60 to 70% of a frog’s body fluids may be frozen. Cardiac function stops and blood ceases to circulate. As their bodies thaw, their hearts resume contractions and their livers rapidly clear the high levels of glucose from the blood even before all of the body’s ice is melted. Thawing frogs display some short term movement and behavioral difficulties but are soon fully functional. Freezing for periods longer than one month, though, leads to rising levels of tissue and organ damage in the frogs. The longer a frog is frozen, then, the greater the magnitude of over-all tissue damage and thus the greater the probability of hibernational fatality.
As soon as the wood frog thaws, it moves to its breeding pools. The isolated melt and rainwater pools it favors are typically free of fish and other potential egg and tadpole predators, but they are also inherently transient and dependent upon unpredictable weather conditions. The use of these temporary pools, then, represents a very delicate, ecological “cost-benefit” balance for the species.
In the mating pools, males call to females with their “duck-like” songs. An attracted female enters the pool and is quickly grasped on the back by the smaller male (this is called “amplexus”). The male may remain in place on the female’s back for 24 to 72 hours. The male releases sperm into the pool water as the female ovulates and thus externally fertilizes the forming egg mass. A typical egg mass contains 1000 to 2000 eggs. The female moves the floating egg mass into the shallow areas of the pool in a large, communal raft. Counting these rafts in an area’s pools is an accepted, and highly efficient, way to determine the population density of the wood frog in a particular region.
Since mating and egg laying occur very soon after ice melt, the chance of seasonal, sub-zero temperatures re-occurring is quite high. The eggs and embryos of R. sylvatica have an interesting adaptation that enable them to survive both transient and sustained periods of freezing and sub-freezing temperatures. The melting point (i.e. temperature at which material changes from a solid (frozen) to liquid state) of the mucopolysaccharide and mucoprotein “jelly” that surrounds the eggs and the developing embryos is higher than that of the fluids inside the egg. As temperatures fall, then, the jelly freezes before the egg or embryo. This freezing osmotically draws water out of the egg into the jelly mass. The dehydrated egg and embryo, then, are more resistant to freeze damage and are thus able to better survive the early spring temperature fluctuations. Larger embryos in particular, are more tolerant of longer periods of freezing, so severe weather patterns may generate a selection pressure for faster growing, and, thus, larger and more resistant embryos. It takes one and a half to two months for the embryos to fully develop into free swimming tadpoles.
So, we saw a glimpse of spring on Saturday! A glimpse of the start of spring, anyway. I am going to ignore the weather forecast for tomorrow. Just another April Fools joke!